Prior Art
A continuing demand exists for the conversion of isobutane and light olefins into high octane motor fuels. The term "HF alkylation commonly refers to the alkylation of isobutane with propylene, butenes and amylenes using a hydrofluoric acid (HF) catalyst. HF alkylation has provided a highly successful method for the production of high octane motor fuels. Despite a long history of safe operation, recent concerns over the possibility of a catastrophic release of HF acid from HF alkylation units has prompted the investigation of modifications or alternatives to the HF alkylation process for the production of motor fuels. One existing alternative is a similar alkylation process that uses sulfuric acid as the catalyst. While the use of sulfuric acid may decrease the degree of the hazard that some associate with the use of HF acid, sulfuric acid processes are still perceived as possibly presenting the same hazard and are not as economically advantageous as the HF alkylation process. Therefore, processing substitutes for the HF alkylation process are still sought.
Other methods of combining isobutane with light olefins to produce motor fuels are known and practiced. However, they do not produce the same quality gasoline products or they are more expensive to install and operate. One such alternative is the dehydrogenation of butanes and the oligomerization of the resulting olefins to produce gasoline boiling range hydrocarbons. The oligomerization of light olefins into higher molecular weight motor fuels using a solid phosphoric acid is well known and its use predates the HF alkylation process. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Patents disclosing the dehydrogenation of a light paraffin stream with oligomerization of the dehydrogenation effluent include U.S. Pat. No. 4,393,259, U.S. Pat. No. 5,049,360, U.S. Pat. No. 4,749,820, U.S. Pat. No. 4,304,948 and U.S. Pat. No. 2,526,966.
It is also known to hydrotreat the olefinic hydrocarbon streams produced by oligomerization to saturate olefins. Patent GB 2186287 discloses dehydrogenation of a C.sub.4 fraction and oligomerization of the resulting olefins to produce a jet aircraft fuel that is optionally hydrogenated to produce premium gasoline. The hydrotreatment of jet fuels, diesel fuels, and lubes produced by dehydrogenation and oligomerization of light paraffins is disclosed in U.S. Pat. No. 4,678,645. Hydrotreating is known to be particularly beneficial when saturating isooctenes to isooctane gasoline. However, hydrotreating is not beneficial for all gasoline fractions produced by oligomerization and can lower octane ratings.
Inefficiency in the production of gasoline boiling range hydrocarbons by the dehydrogenation of butanes stems from the interrelationship of the oligomerization reaction zone and the dehydrogenation zone. Operating the oligomerization zone at conditions that will maximize the conversion of C.sub.4 olefins and other olefins degrades the octane quality of resulting oligomers. Maintaining conversion of the C.sub.4 olefins that enter the oligomerization zone somewhere between 70% to 80% not only results in a high quality oligomerization product, but also a lot of unconverted C.sub.4 olefins. These unconverted C.sub.4 olefins are in addition to the butanes that are present in the feed to the oligomerization zone in proportions of 5 to 30 wt. %. The unconverted C.sub.4 olefins are not readily separated from paraffins and are recycled together to the dehydrogenation zone after saturation of the entire stream to hydrogenate the unconverted olefins and to prevent the formation of dioolefins in the dehydrogenation zone. The passage of additional large quantities of C.sub.4 olefins through both the saturation and dehydrogenation zones reduces the efficiency of the dehydrogenation zones and increases the cost of operating an integrated process.
The reaction of olefins to produce ethers using catalytic distillation has been practiced.
Methods for catalytic distillation are disclosed in U.S. Pat. No. 5,399,787 issued to Ozmen et. al. and U.S. Pat. No. 5,395,981 issued to Marker.
It is an object of this invention to provide an efficient alternative to HF alkylation by reducing the amount of C.sub.4 olefins that is recycled in the integration of a dehydrogenation zone and an oligomerization zone.
It is a further object of this invention to provide an integrated process for the combination of isobutane with light olefins that advantageously produces a comparable motor fuel product in a uniquely integrated series of dehydrogenation, oligomerization, and saturation steps.